Worm gears are used, for example, in electromechanical power steering (EPS) systems in motor vehicles, which reduce the force needed to operate a steering wheel when the vehicle is stationary or when driving at low driving speeds. The EPS assists the driver in steering by using an electric motor to augment the steering force applied by the driver or by using the electric motor to superimpose steering angles. The electric motor thus superimposes a general servo assist on the mechanical steering movement of the driver.
Various configurations of electromechanical steering systems are known. Differences in the various configurations relate to the positioning of the servo unit (e.g., motor and/or control module) and the design of the reduction gear. For example, column electromechanical power steering systems (C-EPS) and pinion electromechanical power steering system (P-EPS) are known. In both C-EPS and P-EPS configurations, a mechanical steering movement of the driver is transmitted to the electric motor, via a worm shaft, by a worm wheel operatively connected to the steering wheel. The worm wheel and the worm shaft can form a unit referred to as a worm gear.
To accommodate forces produced during operation of the worm gear and to reduce noise (e.g., to prevent or least reduce sound and vibrations perceptible to passengers within the vehicle, which is referred to as “noise, vibration, and harshness” (NVH) of a vehicle), the worm shaft and the worm wheel can be formed of different materials. For example, the worm shaft can be made of metal, while the worm wheel can be made of plastic. However, a worm wheel formed of plastic may wear more rapidly than the counterpart worm shaft formed of metal. Over the life of the worm gear, the meshing engagement of the teeth of the worm wheel with the worm shaft will vary, thereby increasing tooth backlash and accompanying rattle and vibration noise.
Another disadvantage of using plastic parts coupled to metal parts in worm gears is that fluctuations in temperature and relative humidity can affect the engagement between parts. For example, the plastic and metal parts may have significantly different coefficients of expansion and/or coefficients of water absorption. Temperature and humidity variations may thus produce different effects in the worm shaft and the worm wheel, for which current configurations may not be able to compensate. These environmental effects can also lead to rattle and vibration noise, as well as the potential for jamming or lockup of the worm gear. In addition, when reinforced plastics are used for the worm wheel, even greater amounts of expansion may occur for a given temperature fluctuation which can lead to a distortion in the worm gear. Reinforced plastics may also be susceptible to increased swelling via absorption of water from the air.
To allow for greater design tolerances during manufacturing and dimensional variations in the worm gear, it may be desirable to employ a low spring constant and/or low spring force for the worm gear. However, such a design may be at odds with an optimal operation of the worm gear, in particular, to achieve acceptable NVH behavior. For example, rattling can arise from poor intermesh engagement between the worm wheel and the worm shaft due to the lower spring force/constant when a torque from the steering wheel acts on the worm wheel and a radial force is applied to the worm shaft. Thus, a higher spring force may be desirable in the tooth engagement area.
In a worm gear, a worm shaft can be connected via a coupling to a drive shaft, which carries the rotor of an electric motor. The worm wheel and the worm shaft of the worm gear can be brought into pre-tensioned engagement. For example, pretensioning can be achieved using a radially acting spring, which acts against the tooth engagement area of the worm gear and thus adjusts the tooth backlash to ensure engagement of the worm wheel to the worm shaft with little or no play. However, since the torque of the electric motor has to be transmitted to the worm shaft via the drive shaft, one should compensate for the manufacturing design tolerances between the drive shaft and the worm shaft. In addition, space may be required outside the worm gear to accommodate the spring or another element to pretension the worm wheel against the worm shaft.
With this in mind, the object of the present disclosure is to provide a worm gear that allows for large design tolerances during manufacturing with improved NVH performance and reliability.